Small fiber neuropathy (SFN) is a neurodegenerative disorder that is characterized by a loss of unmyelinated C-, small myelinated Ad- and autonomic fibers, and is associated with sensory and autonomic dysfunctions, including burning, pin-prick and lancinating pain and autonomic symptoms, which are often refractory to treatment. SFN has been linked to multiple causes, including diabetes, alchoholism, toxins, including Agent Orange, HIV, anti-neoplastic treatments, and autoimmune disorders, but a substantial percentage of SFN are idiopathic, with no apparent cause. We have recently demonstrated the presence of gain-of-function variants in sodium channel Nav1.7 in nearly 30% of patients with SFN and no other apparent cause. However, the molecular cascade leading to small fiber loss and onset of pain associated with the gain- of-function variants in Nav1.7, or any of the multiple causes linked to SFN, is not yet understood. The proposed Small Project is designed to develop an in vivo model of small fiber neuropathy by creating knock-in mice with heterozygous insertion of a human gain-of-function variant in the Scn9a (Nav1.7) gene at the Scn9a allelle. Multiple gain-of-function variants in Nav1.7 have been found in patients with idiopathic SFN, and our electrophysiological and morphological studies of two variants (I228M and G856M) as exceptionally promising candidates in which to pursue generation of in vivo models. For our studies, knock-in mice are generated and phenotypically characterized by morphological, whole-cell patch clamp and behavioral methods. In future studies not funded by this proposal, the knock-in mice will serve as valuable platforms in which to perform mechanistic studies to determine the molecular pathway(s) leading to axonal degeneration and the onset of sensory and autonomic dysfunction in SFN. It is also anticipated that the development of the in vivo models will allow us to identify and test novel therapeutic approaches to minimize axonal degeneration, and the sensory and autonomic dysfunctions associated with fiber loss, in human small fiber neuropathy.